Stents are frequently used in the medical field to open vessels affected by conditions such as stenosis, thrombosis, restenosis, vulnerable plaque, and formation of intimal flaps or torn arterial linings caused by percutaneous transluminal coronary angioplasty (PCTA). Stents are used not only as a mechanical intervention, but also as vehicles for providing biological therapy. As a mechanical intervention, stents act as scaffoldings, functioning to physically hold open and, if desired, to expand a vessel wall. Stents may be capable of being compressed in diameter, so that they can be moved through small vessels with the use of a catheter or balloon-catheter, and then expanded to a larger diameter once they are at the target location. Accurate positioning of the stent at the target location involves the use of fluoroscopy, which relies on the stent being radiopaque to distinguish it from surrounding tissue and from the delivery device, such as a catheter, on which it is being carried. Examples of such stents include those described in U.S. Pat. No. 4,733,665 to Palmaz, U.S. Pat. No. 4,800,882 to Gianturco, U.S. Pat. No. 4,886,062 to Wiktor, U.S. Pat. No. 5,514,154 to Lau et al., and U.S. Pat. No. 5,569,295 to Lam.
A stent must have sufficient radial strength to withstand structural loads, such as radial compressive forces, imposed on the stent as it supports the walls of a vessel or other anatomical lumen. In addition, the stent must possess sufficient flexibility to allow for crimping, deployment, and cyclic loading from surrounding tissue. Also, a sufficiently low profile, that includes diameter and size of struts, is important. As the profile of a stent decreases, the easier is its delivery through an anatomical lumen, and the smaller the disruption in the flow of blood or other bodily fluid.
FIG. 1 shows an end segment of an exemplary stent 10 designed to be crimped onto a catheter and subsequently expanded. Stent 10 has a cylindrical shape having central axis 12 and includes a pattern of interconnecting structural elements or struts 14. Axis 12 extends through the center of the cylindrical shape formed by struts 14. The stresses involved during compression and deployment are generally distributed throughout various struts 14. A surface coating of material may be applied over the struts 14. As opposed to the surface coating of material, it is the underlying structure or substrate material of struts 14 that is typically the primary source of the radial strength of stent 10.
There are different types of struts 14. Struts 14 include a series of ring struts 16 that are connected to each other by bending elements 18. Ring struts 16 and bending elements 18 form rings 20 configured to be reduced and expanded in diameter. Rings 20 are arranged longitudinally and centered on axis 12. Struts 14 also include link struts 22 that connect rings 20 to each other. Rings 20 and link struts 22 collectively form a tubular scaffold of stent 10.
Bending elements 18 bend to a more acute angle when stent 10 is crimped to allow radial compression of stent 10 in preparation for delivery through an anatomical lumen. Bending elements 18 subsequently open to a larger angle when stent 10 is deployed to allow for radial expansion of stent 10 within the anatomical lumen. After deployment, stent 10 is subjected to static and cyclic compressive loads from surrounding tissue. Rings 20 are configured to maintain the expanded state of stent 10 after deployment.
Stents made of bioresorbable polymers have been developed to allow for improved healing of the anatomical lumen. Examples of bioresorbable polymer stents include those described in U.S. Pat. No. 8,002,817 to Limon, U.S. Pat. No. 8,303,644 to Lord, and U.S. Pat. No. 8,388,673 to Yang. Because polymers are generally less radiopaque than conventional metals used for stents, metallic radiopaque markers are attached to parts of the stent to allow for visualization and accurate positioning using fluoroscopy.
FIG. 2 shows two metallic radiopaque markers 30 imbedded side by side in link strut 22 between two rings 20. Radiopaque markers 30 are spheres approximately 240 μm in diameter which have been press-fitted into slightly undersized holes of approximately 230 μm diameter. Substrate material 32 forms the perimeter of the holes and extends continuously from one ring 20 to the adjacent ring 20. It is substrate material 32, not radiopaque markers 30, which maintains the connection between rings 20. Rings 20 would remain directly and structurally connected to each other via link strut 22 even when radiopaque markers 30 are removed from the stent.
FIG. 3 shows a fluoroscopic image of a bioabsorbable polymer stent being deployed in a vessel. Two radiopaque markers are imbedded as shown in FIG. 2 at opposite ends of the stent. The radiopaque markers are 240-μm diameter spheres made of platinum. As these markers are close together, they can appear as a single marker depending on the resolution of the fluoroscope. The stent is carried on a balloon catheter which has its own radiopaque markers which demark the edges of the stent or working length of the balloon. The dark spots corresponding to the radiopaque markers on the stent are not as visible as the radiopaque markers on the balloon. The radiopaque markers on the balloon were more visible because they were larger than those on the stent. After the balloon is withdrawn, all that remains visible under fluoroscopy of the bioresorbable stent are the radiopaque markers. For a skilled interventionalist, this can be adequate for locating the bioresorbable stent. However, with patients of larger mass (longer x-ray path length), suboptimal fluoroscopy equipment, or a physician less experienced with the bioresorbable stent, locating the bioresorbable stent can be challenging.
One approach to making the stent more visible is to increase the number of radiopaque markers. As can be imagined from FIG. 3, imbedding more 240 μm markers in line along a thin stent strut will increase the number of faint spots, as opposed to creating a single dark spot that is easily distinguished from its surroundings. Accordingly, this approach is likely to provide little improvement in distinguishing the radiopaque markers on the stent from surrounding structures and from the substantially larger radiopaque markers on the balloon.
Accordingly, there is a continuing need for making polymer structures of a stent more radiopaque without impacting the ability of the bioresorbable stent to meet the mechanical and functional requirements discussed above.